"Animorphs" is a science fiction book series created by K.A. Applegate that follows a group of teenagers who can transform into animals. They acquire this ability after encountering an alien named Elfangor, who gives them the power to morph in order to fight against the parasitic alien race known as the Yeerks. The series explores themes of identity, responsibility, and the impact of war. The main characters, including Jake, Rachel, Tobias, Cassie, and Marco, face various challenges as they navigate their dual lives as ordinary teens and secret warriors. Each book features their adventures, moral dilemmas, and the consequences of their choices in the battle to protect Earth from the Yeerk invasion.